In general, friction welding involves moving two workpieces resting against one another relative to one another, the friction which arises leading to heating and plasticization of the material of the workpieces in the friction zone. If the two workpieces are placed or pressed against one another, the plasticized material of the two workpieces bonds in the region of contact. One of the special advantages of friction welding is the fact that a large number of different materials can be joined together and that the region of the workpieces which is affected by the welding process is relatively small and is exposed to comparatively low temperatures.
Starting from this basic principle, modifications of the friction welding process have been developed. In friction stir welding, for example, frictional energy is produced by a wear-resistant rotating tool which moves through the material of two workpieces to be joined along or in the vicinity of the bearing surface between them, with the result that the material is plasticized along or in the vicinity of the path of motion of the tool, this in turn making possible the joining of the two workpieces in this region.
The prior art, e.g. WO 01/28732 A1 discloses a friction welding apparatus which has a pin that is driven in rotation about its longitudinal axis and has a coupling end and an engagement end lying opposite the coupling end, wherein the coupling end is rigidly connected to the output of the friction welding apparatus and the engagement end is provided for engaging with the surface of two workpieces placed against one another and penetrating into the material of said workpieces. This publication furthermore discloses the provision of a clamping device on the friction welding apparatus, said device surrounding the pin and being capable of movement relative to the pin along the longitudinal axis of the latter in order to exert pressure on the workpieces. The clamping device is mounted on the machine frame of the friction welding apparatus and can thus be moved independently of the pin.
The disadvantage with a prior art friction welding apparatus of this kind, however, is that it cannot be used in any other way as a machine tool, e.g. for drilling or milling, since at least the clamping device is a fixed part of the machine tool and cannot be detached from the latter. On the other hand, however, a clamping device is indispensable since the workpieces would otherwise be able to move relative to one another during the welding process or could form unwanted gaps.